How to draw a cylinder with an irregular cross section, which varies along the cylinder length?
If you need to work in micrometers, it is better to select None as the Length unit and just keep in mind that the scene unit is micrometer, because Blender starts malfunctioning  if you try to scale units to that size even if you do that with unit scale value:



It is easier to create the cylinder then it may seem. We could use vertex slide functionality(shift+v) to quickly and easily specify the lengths of the radii. Vertex slide works proportionally to the edge it slides on and we want to enter units instead of proportions during the slide operation, so let's adjust the proportion to match the units by making a circle that would have a radius of 1 unit. Once you have the circle, enter edit mode (tab) select the edge loop and extrude it(e), scale it in a bit (s), extrude again and merge the loop to center with alt+m. Now you can take the vertices one by one. Slide every  vertex (shiftvv) to the center, slide it again, hit c to clamp it to the path so it can go further than the edge loop marking 1 unit, and enter the units before confirming the operation with a mouse click. Repeat this for all vertices around, once you are done, you can delete the 1 unit mark edge loop, select everything and extrude: